Matthew During wanted to see whether stressing out mice by messing with their environment would affect the rate of tumor growth. So, for a study that now appears in Cell, he and his team divided up their mice into two groups. Some mice lived quiet, peaceful lives in cages shared between five mouse roommates, while the other group lived in a stressful cluttered cacophony, where the cages held 18 to 20 animals plus numerous distractions and challenges like toys, mazes, and wheels.

Mice were then injected with tumor cells, which led to malignancies in all of the control animals within 15 days… The rate of tumor formation in animals living in the enriched environment was significantly delayed, and 15 percent had not developed tumors after nearly three weeks; when tumors were visible, they were 43 percent smaller than the lesions on control animals [Scientific American].

Because the “enriched environment” gave those mice so much more to do, an obvious conclusion would be that it’s the uptick in physical activity—not the effect of added stress—that kept tumors at bay. So During’s team tested the mice to see if just giving them more time on the running wheel, independent of the other factors, was enough to see the effect. It wasn’t.

Physical exertion alone also didn’t inspire the chemical changes that the team thinks could be responsible for the anti-cancer effects they saw.

The ‘enriched’ mice, the researchers found, had slightly raised levels of stress hormones, but the most striking physiological change was markedly reduced levels of the hormone leptin, known to regulate appetite. Blocking leptin abolished the effects of enrichment, suggesting that the hormone was key to the pathway that led to the anti-cancer effects [Nature].

The linchpin behind all this seems to be a protein called brain-derived neurotrophic factor (BDNF), which increased in the enriched-environment mice. Further tests by the team suggested that the protein spurred the drop in leptin and boosted the immune system, which would be connected to the tumor reduction.

That’s fine for mice, but what about us?

Whereas most people live in fairly safe environments, with plenty of food and some degree of social interaction, “our data suggests that we shouldn’t just be avoiding stress, we should be living more socially and physically challenging lives,” During says [Scientific American].

Some stress is good, some is bad. Stress is a response by the body to certain stimuli that changes the normal function of an organism. Sometimes this change is a good thing, leading to feelings of excitement, enthusiasm, and accomplishmnet. Sometimes it’s a bad thing, leading to fear, anxiety, and worry. The good stress is called eustress, the bad stress is called distress.